Recently, interest has been expressed in environmental issues and this situation necessitates strengthened and thinner automobile steel sheets enabling mileage improvement due to their lighter weight. Although 440 MPa grade steel is most frequently used for high strength hot rolled steel sheets today, sheets of 490 MPa or higher grade steel, in particular, 590 MPa grade steel, have been increasingly used for the reason described above. However, such strengthening also reduces ductility and stretch-flangeability, thereby posing problems such as formation of cracks in press working and a decrease in the yield.
Meanwhile, recent advancements in press technology have resulted in growth in the number of applications of working processes including drawing or stretch forming, piercing, and subsequent flange forming at sites of stretch flange deformation. Steel sheets formed by such processes will then be worked, and thus have to maintain stretch-flangeability even after piercing. However, no 490 MPa or higher grade steel sheets that support such a new working method have been developed thus far.
As a technique for improving the stretch-flangeability of unworked steel sheets, a technique wherein a slab to which Si has been added is heated at a temperature of 1200° C. or lower, hot rolled, rapidly cooled to a prescribed temperature, cooled at room temperature, and then coiled at a temperature in the range of 350 to 550° C. to produce a phase consisting mainly of bainite is disclosed in Japanese Unexamined Patent Application Publication Nos. H04-088125 and H03-180426. However, in those techniques, the temperature for heating the slab should be suppressed to prevent the formation of red scales due to the addition of Si and this would pose problems such as an increase in rolling forces and deterioration of surface characteristics. Furthermore, a phase consisting mainly of bainite would also be problematic in terms of stretch-flangeability after working.
Japanese Unexamined Patent Application Publication No. H08-325644 discloses a technique for manufacturing a steel sheet that is stable in terms of material characteristics within a coil and excellent in terms of stretch-flangeability, with an emphasis being placed on the first half of cooling, wherein cooling at a temperature of 540° C. or lower is performed as slow cooling (the cooling rate is small and in the range of 5 to 30° C./s), and cooling is performed in the film boiling region. However, cooling at a temperature of 500° C. or lower, in particular, 480° C. or lower, using film boiling necessarily leads to an increase in localized temperature unevenness that emerges in the preceding cooling steps (e.g., localized cooling due to water retention caused by defects in the shape), and as a result, localized variation of material characteristics within a coil may occur. Additionally, a slow cooling rate would promote ferrite transformation in a portion of the phase during cooling, thereby making it difficult to control the fractions of ferrite and bainite. As a result, the stretch-flangeability after working is insufficiently improved. Moreover, there would be an additional problem in terms of equipment, i.e., the line length of the cooling line has to be long.
Japanese Unexamined Patent Application Publication No. H04-276042 discloses a technique for obtaining a steel sheet with totally well-balanced strength, yield ratio, stretch-flangeability, and other characteristics, wherein a material is rolled by 70% or more in a finishing rolling step, very rapidly cooled at a rate of 120° C./s or higher, and maintained at a temperature in the range of 620 to 680° C. for 3 to 7 seconds to provide a fine ferrite phase, and then the fine ferrite phase is further cooled at a cooling rate in the range of 50 to 150° C./s and coiled at a temperature of 400 to 450° C. However, in this technique, a large pressure used in the finishing rolling step often results in surface defects and the very rapid cooling after hot rolling deteriorates the shape of a resulting steel sheet. Cooling a steel sheet having a deteriorated shape at a cooling rate of 50° C./s or higher to a temperature of 480° C. or lower would increase unevenness of cooling in some sites, thereby posing a problem of localized variation of material characteristics.
In addition, Japanese Unexamined Patent Application Publication No. 2000-042621 discloses a technique for controlling cooling of a thick steel sheet produced without a coiling step. That technique is intended to reduce the hardness difference between the surface layer and the inside of a thick steel sheet, which is caused by unevenness of cooling or other factors, by using only film boiling in the first half of cooling and using only nucleate boiling in the second half of cooling, thereby preventing the variation of material characteristics of the thick steel sheet. However, that technique is applied to a thick steel sheet having a thickness of 10 mm or larger, and thus is difficult to apply to a thin steel sheet that is produced with a coiling step and is mainly applied to have a thickness smaller than 10 mm and typically equal to or smaller than 8 mm.
Therefore, in hot rolled steel sheets (hot rolled steel bands) produced by coiling, it is difficult to eliminate the variation of material characteristics while maintaining desired characteristics merely by eliminating unevenness of cooling that occurs after hot rolling. It is thus necessary, for example, to establish a steel phase having desired characteristics while taking into consideration the component composition of the steel as well as the influences of the pattern for the cooling step performed after hot rolling and the temperature for the subsequent coiling step.